Corrections of indices of abundance of 0-group fish in the Barents Sea for varying capture efficiency

Each year since 1965 "The International 0-group Survey in the Barents Sea and Adjacent Waters" has provided indices of abundance of 0-group fish. During the 30 years of operation of the survey large improvements in capture efficiency have been introduced including larger trawls, instrumentation for monitoring trawl depth and geometry and larger and better equipped vessels. The time series of survey indices indicate an increasing trend throughout the period and thus it seemed reasonable that the improvements have effected the level of the abundance indices. The present work is an attempt to correct the indices of abundance so that the comparability of indices from different parts of the time series is maintained. Corrections were made following two lines of approach: 1. Assuming that the annual average capture efficiency had increased proportional to the increase of the average area ofthe trawl mouth (opening). 2. Assuming that the main trend in the sum of indices of cod, haddock and redfish reflects the trend in overall capture efficiency. Both methods generated a substantial increase in the indices for the early part of the time series